When producing oil and/or gas from an unconsolidated subterranean formation, some type of particulate control procedure may be required in order to prevent sand grains and/or other formation fines from migrating into the well bore and being produced from the well. The production of such particulate materials can reduce the rate of hydrocarbon production from the well and can cause serious damage to well tubulars and to well surface equipment.
Those skilled in the art have commonly used gravel packs to control particulate migration in producing formations. A gravel pack will typically consist of a mass of sand and/or gravel which is placed around the exterior of a screening device, said screening device being positioned in an open hole or inside a well casing. Examples of typical screening devices include wire-wrapped screens and slotted liners. The screening device will typically have very narrow slots or very small holes formed therein. These holes or slots are large enough to permit the flow of formation fluid into the screening device but are too small to allow the gravel/sand constituents of the gravel pack to pass therethrough. In conjunction with the operation of the holes or slots formed in the screening device, the gravel/sand constituents of the gravel pack operate to trap, and thus prevent the further migration of, particulate materials which would otherwise be produced along with the formation fluid.
Unfortunately, the installation of gravel packs in underground formations can be quite costly. Additionally, special equipment is required for installing gravel packs.
Another technique used to control particulate migration in producing formations involves the use of chemical consolidation treatments. Chemical consolidation treatments can also be quite costly. Further, these treatments require the use of special chemicals and equipment.
Nonpermeable foamed cement compositions have been used heretofore in oil and gas wells for performing various primary cementing operations. Nonpermeable foamed cement compositions are formed by introducing nitrogen, air, or some other gas into a cement slurry. Compared to non-foamed cement compositions, nonpermeable foamed cement compositions typically have low densities and low fluid loss properties.
In performing a primary cementing operation using either a nonpermeable foamed cement composition or some other type of nonpermeable cement slurry composition, the cement composition is pumped down a casing disposed in a well bore such that, when the cement slurry reaches the bottom of the casing, the cement slurry flows up and into the annulus existing between the exterior of the well casing and the earthen wall of the well bore. Upon setting, the nonpermeable cement composition bonds to the casing and to the well bore such that (1) the casing is rigidly supported within the well bore and (2) fluid flow within the cemented portion of the annulus is prevented.
Due to their low densities, nonpermeable foamed cement compositions can be advantageously used in primary cementing operations where it is necessary to minimize hydrostatic pressure effects on weak formations and/or to lift primary cement columns over long annular intervals. Additionally, compared to nonfoamed cement compositions, nonpermeable foamed cement compositions typically have high compressibilities. Due to their high compressibilities, nonpermeable foamed cements are resistant to the incursion of pressurized formation gases into and around the cement composition during the primary cementing operation (i.e., before the cement composition has set).
As is well known in the art, a high deviation well, e.g., a horizontally completed well, can be drilled when it is desirable to obtain a well bore which is not strictly vertical. As used herein and in the claims, the term "high deviation well" refers to any well having a well bore which is intentionally drilled such that one or more portions of the well bore are nonvertical. A high deviation well bore can be drilled, for example, when it is desirable to direct the well bore around, to, or through a given formation. The term "horizontally completed well," as used herein, refers to a well wherein the well bore has been drilled to include one or more substantially horizontal sections.
Subterranean formations, although typically very thin, can extend great distances horizontally. Thus, although the bore of a strictly vertical well would extend only a few feet through a typical thin formation, a horizontally completed well can include one or more horizontal well bore sections which extend several hundred or several thousand feet through the formation. By providing much greater contact between the well bore and the formation, the horizontally completed well can provide a higher production rate than would be provided by a strictly vertical well.
In one technique commonly used for completing high deviation wells, a casing is installed in only the substantially vertical initial portion of the well bore. Consequently, formation fluid flows freely into the uncased horizontal portion of the well and is then recovered through the vertical well casing. Unfortunately, however, the uncased horizontal portion of the well will typically be highly susceptible to cave-ins and sloughing, particularly when the formation through which the horizontal section of the well bore runs is a significantly unconsolidated formation. Additionally, the level of particulate migration occurring in the uncased horizontal portion of a horizontally completed well can be quite high. As discussed above, particulate migration can reduce the hydrocarbon production rate from the well and can cause serious damage to well tubulars and surface equipment.
A second technique commonly used for completing horizontal wells involves placing a length of slotted liner or casing in the horizontal portion of the well. The slotted liner or casing operates to prevent the horizontal portion of the well from collapsing. In order to prevent particulate migration into the slotted liner or casing, a gravel pack can be placed around the exterior of the liner or casing in the same manner as described hereinabove. However, as also discussed above, the installation of a gravel pack can be quite costly, particularly when the gravel pack must extend several hundred or several thousand feet along the horizontal portion of a horizontally completed well.
A third technique commonly used for completing horizontal wells involves placing and cementing a casing in both the vertical and horizontal portions of the well bore. Perforations or sliding sleeve valves must be placed along the horizontal portion of the casing in order to allow the casing to communicate with the producing formation. The formation is typically fractured through these casing perforations or valves. Unfortunately, however, this system typically does not provide adequate protection against the migration of formation particulates into the well casing. Additionally, the perforating operation itself may promote the deconsolidation of the formation. Further, in a highly deviated well, it is typically not desirable to place cement across the productive interval of a naturally fractured formation since the cement will block the horizontal flow of fluid from the natural fractures to the casing perforations or valves.
In view of the above comments, it is evident that a problem of longstanding existing in the completion of wells in subterranean formations having a substantial degree of unconsolidation resides in the need to reduce, if not prevent, the migration of formation particles from the formation to the production tubing and surface equipment without, at the same time, reducing the flow of desired fluids, e.g., oil and/or gas, from the formation. This problem is difficult enough when the borehole is substantially vertical, but it is even more difficult when the borehole is highly deviated or is, in fact, horizontal.
It is understood that producing formations cannot be blocked, such as by primary cementing, because cements ordinarily employed in primary cementing, have very low permeability, e.g. less than about 0.001 darcies, which would prevent the flow of desirable fluids from the formation to the production equipment. Accordingly, producing formations penetrated by a borehole are usually not cemented and migration of formation particulates from unconsolidated formations is reduced, or prevented, as above discussed, chemically, by employing a formation consolidation technique, or, mechanically, by employing a gravel packing technique. The above techniques have been used successfully in completing a substantially vertical borehole wherein the portion of the borehole which does not penetrate a producing formation can be cemented to thereby support the casing and isolate and protect producing formations, while unconsolidated producing formations penetrated by the same borehole can be chemically or mechanically treated, as mentioned above, to reduce or prevent fines migration whole not blocking the flow of desirable fluids.
In contrast with a vertical borehole, a borehole, or a very long portion of one, which lies entirely within a producing formation, such as a horizonal borehole, requires the use of a completion technique which will function to maintain the structural integrity of the borehole itself, i.e., prevent collapse, which will not prevent the flow of desirable fluids from the formation to the production tubulars. Known cements would maintain structural integrity of the borehole, but would also prevent flow of desired fluids. Chemical and mechanical treatments, as above described, would not prevent the flow of desired fluids, but are very difficult to install in highly deviated boreholes and the ability of such treatments to provide adequate structural integrity has not been established.
Accordingly, the art requires a method which will supply the structural integrity provided by primary cementing; which will control, where required, the movement of formation fines; and which will not prevent the flow of desired fluids from the formation to the production tubulars. This invention provides such a method which features the use of a cement having a permeability low enough to prevent migration of formation particulates but high enough to permit the flow of desired fluids through the hardened cement to production tubulars. The cement develops sufficiently high compressive strength to support and protect formations but is also of sufficiently low density to permit use in weak formations. This cement and the method disclosed is, accordingly, useful in vertical as well as in highly deviated and horizontal boreholes.